The present invention relates to the art of image artifact correction. It finds particular application in conjunction with removing out-of-slice artifacts from critical regions of an imaging volume in single signal average scans offset in the phase encode direction and will be described with particular reference thereto.
Heretofore, medical diagnostic images have commonly included ghosts or artifacts from various sources including edge information, eddy currents, and signals emanating from out of the selected slice.
One technique for removing these artifacts was to place symmetric spoiler gradients around the 180.degree. refocusing pulse. The symmetric gradients had the effect of selectively dephasing signals originating from the 180.degree. pulse itself. See, for example, U.S. Pat. No. 4,484,138 to Bottomley, et al. Although effective in removing artifacts originating from the 180.degree. refocusing pulse, the application of the symmetric gradients had numerous disadvantages. First, these gradients, which were added solely for artifact elimination, increased the time required to perform an imaging sequence. The time used to eliminate the artifacts increased the minimum echo time which the imaging system could attain. Increasing the minimum echo time leads to decreased signal to noise and may reduce the total number of slices which can be acquired in a given repeat time, i.e. reduces slice throughput. The time required for the symmetric gradients shortens the allowable read gradient time for short echo times. This decrease in the read gradient time limited the minimum achievable field of view and decreased the sequence signal to noise ratio. The application of the symmetric gradients increased the RMS current and power dissipation requirements in the gradient amplifiers, reducing slice throughput. Moreover, the symmetric gradients do not totally eliminate the out-of-slice artifacts. Residual out-of-slice artifacts still overlap the region of interest.
Many of the disadvantages of the spoiler gradient technique can be overcome by estimating the out-of-slice artifact and subtracting it from the data set. That is, a data set is generated from the sample without using phase encode gradients. Without phase encode gradients, the resultant data set should form a blank image. However, the resultant "blank" image includes out-of-slice artifacts, such as those artifacts attributable to eddy currents induced by the imaging sequence. This technique is described in greater detail in U.S. Pat. No. 4,959,611 to the inventors herein. Although curing many of the drawbacks of this gradient spoiling technique, this technique still has some drawbacks. First, the phase encode gradients themselves cause eddy current changes. This changes the artifacts in the actually collected image relative to the "blank" image. When the "blank" image or correction data is subtracted, the out-of-slice artifacts do not cancel completely. Because the out-of-slice artifact estimate is always imperfect, residual artifacts will continue to occur and these artifacts often overlap the region of interest in the imaging volume. That is, the artifacts or ghosts tend to be superimposed at least in part on the region of data that is being analyzed.
Prior art analog radio frequency transmitters commonly had a DC component or offset. This DC offset was removed from the resultant magnetic resonance data by alternating the phase of the RF excitation pulse by 180.degree.. That is, in odd number sequence repetitions, for example, the RF excitation pulse had a +90.degree. phase offset and in even numbered sequence repetitions, the RF excitation pulse at a -90.degree. phase offset. This reversed the polarity of the DC offset such that the DC offsets canceled. One side effect to this technique for removing the DC offset is that it shifted out-of-slice artifacts or ghosts one-half a field of view in the phase encode direction relative to the isocenter of the magnetic resonance imager.
Modern magnetic resonance scanners use digital, rather than analog, RF transmitters. The digital transmitters do not introduce a DC component, hence the .+-.90.degree. phase cycling is not needed to eliminate DC offset.
In accordance with the present invention, a new and improved imaging technique is provided in which out-of-slice artifacts are shiftable consistently to a selectable region of the field of view, even when the field of view is translated relative to the isocenter of the imager.